how open is open?

Im wondering, for an open headphone, how much sound escapes and is heard to others not wearing the open-headphone?
is it literally 100% (as noisy on the outside - at point blank range - as it is on the inside), or around 50%, 10%

Obviously an actual percentage is not easy to say, but I'm just wondering how noisy an open-headphone would sound, when played at a fairly loud volume, to someone else sitting on the same couch? Or from 10 feet? Or from another room?

On average it's like a radio playing from a small speaker at low volume. Loud enough that anyone sitting close by watching TV will be bothered by the sound. Also if used at work, it would cause problems for someone sitting in a cubicle close by.

thanks.
Im not concerned about someone sat next to me, as I can use my closed-phones then. But I have 2 babies in the house and I wouldnt want to any sound travelling to their bedroom (closed door but on the same floor). So I just thought it be best to check how much sound escapes.

Normally it won't bother the babies when their door is closed. But I have played Sennheiser 600's in an office when my boss was two offices away with his door closed, and it was so loud it disturbed him. He asked me "doesn't that hurt your ears, when I can hear it that far away?"

And I said "No, they're high fidelity headphones, so realistic sound levels don't cause damage." But they may disturb, so you may have to keep the volume to a modest level.